The National Republican Congressional Committee (NRCC) expects to mount a direct-mail and advertising blitz of selected House Democrats in an effort to soften up incumbents before the 1986 midterm elections.

NRCC Chairman Guy Vander Jagt (R-Mich.) said yesterday that the committee may spend as much as $200,000 in each of 20 to 50 targeted Democratic districts on messages aimed at undercutting the incumbents.

The $4 million to $5 million program is likely to increase deep partisan divisions in the House this year, and Vander Jagt said the White House "has raised the possibility that this may be counterproductive."

"The White House hasn't agreed to it," he said.

Reflecting the fact that Republicans are still smarting from their failure to win more House seats in last November's presidential landslide, Vander Jagt said the new program is needed to overcome "the awesome power of incumbency."

He charged that many incumbent Democrats implied while campaigning last fall that they supported President Reagan, and he warned that his committee would counter with mail and advertising suggesting the opposite.

"There's a little education that needs to be done," he told reporters at a breakfast.

Democratic campaign officials questioned the legality of the GOP blitz under federal campaign spending limits. "I'm always open to creative legal interpretations, but it strikes me they're going to have to come up with a doozy," Martin Franks, executive director of the Democratic Congressional Campaign Committee, said.

Rep. Tony Coelho (D-Calif.), the committee's chairman, said Democrats would respond in kind. "We're going to be out there just as aggressively for their people as they are for ours," he said.

Coelho predicted that the Republican effort will boomerang, noting that the GOP had tried other advertising programs against House Democrats. "Every time they've tried it, they've bombed," he said.

Vander Jagt indicated that this program's size and scope is different. It would begin in April and continue until the summer of 1986 and include direct mail, radio and television ads.

Already, the NRCC is using the contested House race in Indiana's 8th Congressional District to blast Democrats in selected districts.

The decision to launch the program came after research by Republican pollster Robert Teeter showed that many voters who supported Reagan and a Democratic House candidate also strongly favor some of Reagan's programs.

Teeter said the GOP is at parity with the Democrats in measures of party identification and indicated that the new program is part of a long-term Republican effort to chip away at the Democrats' sizable House majority.